Method of forming a bibulous coating on a metallic anode



Rodolfo Rodriguez Balaguer, Union de Reyes, Quba, assignor to .I. D.Hedges and Company Limited Partnership, Matanzas, Cuba, a limitedpartnership of Cuba No Drawing. Application May 23, 1957 Serial No.660,998

14 Claims. (Cl. 117-230) The present invention relates to bibulouscoatings and more particularly to a method for producing such a coatingon a metallic element to be used as an anode in a primary battery.

A number-of materials has been employed as bibulous coatings for drycell anode elements. For example, specially prepared paper materialshave been and are still in wide use for this purpose. A bibulous coatingwith exceptionally desirable characteristics for use in a primarybattery is starch. However, considerable difiiculty has been encounteredin providing suitable starch coatings on anode elements of specialshapes, as a result of which.

paper coatings are often used for structures in which a starch coatingwould be preferred. For example, difficulty has been found in providinga starch coating on anode elements for the so-called inside-on batteriesin which the anode element is inserted into the battery mix previouslyplaced in the carbon cathode.

The present invention is concerned with the provision of starch bibulouscoatings and a principal object thereof has been the provision of anovel and improved method for producing such a coating.

More particularly, it has been an object of the invention to provide amethod for producing a bibulous coating of starch on a primary batteryanode element, which coating has a desired uniform thickness over thesurface area of the element.

An important object of the invention has been the provision of such amethod in which a very thin, tough starch coating is provided.

Another object of the invention has been the provision of such a methodwhich is susceptible of use in the modcm high speed, low cost operationsemployed in manufacturing primary batteries.

Still another object of the invention has been the provision of such amethod in which the coated anode element can conveniently be forced intoplace through a mass of battery mix.

A further object of the invention has been the provision of such amethod which will produce a bibulous coating on the inside and outsidesurfaces of a cylindrical anode structure.

Other and further objects, features and advantages of the invention willappear more fully from the following description of the invention.

The method, in accordance with the invention, involves the provision ofa starch solution which preferably con- I tains a quantity of theelectrolyte to be incorporated in Patented July 7, 1959 annularcylindrical anode construction described in my copending applicationSerial No. 572,085, filed March 16, 1956. The anode constructiondescribed therein, of which a number of modifications is suggested, isan excellent example of a structure on which it would be difiicult ifnot economically impossible to provide a thin starch coating by thestarch coating methods of the prior art. In accordance with the presentinvention a uniform, hard, thin starch bibulous coating may be producedon both the inside and outside surfaces of the anode cylinder, theproduction of the starch coating requiring very little time and beingeasily accomplished by mass production techniques.

As previously indicated, the anode element, which will generally be madeof zinc but which may be made of any suitable metal, is heated and isthen immersed into a starch containing solution. The constituents of thesolution and their concentration may be varied over a wide range,depending upon the characteristics desired in the bibulous coating. Inmany cases, and particularly when the bibulous coating to be producedwas a thickness greater than about & inch, it will be desirable toinclude in the solution an electrolyte of the type to be used in thebattery mix provided for the cell being produced. However, this is notessential and an aqueous starch solution may be employed. If the starchcoating is to be very thin, e. g., less than about inch thick, littleadvantage will normally be gained by using an electrolyte solution.

Typical examples of suitable solutions will now be set forth. In eachcase the electrolyte can be replaced with water.

Solution #1 An electrolyte was prepared by dissolving 25 g. of ammoniumchloride (NH Cl) and 10 g. of zinc chloride (ZnCl in 60 cc. of distilledwater. The solution was formed by dissolving 15 g. of wheat starch in 40cc. of the electrolyte.

Solution #2 An electrolyte was prepared by dissolving 25 g. of ammoniumchloride and 15 g. of zinc chloride in 60 cc. of distilled water. Thesolution was formed by dissolving 25 g. of yucca starch in 40 cc. of theelectrolyte.

Solution #3 The solution was formed by dissolving 5 g. of wheat flourand 20 g. of yucca starch in 40 cc. of electrolyte prepared as inSolution #2.

Solution #4 The solution was formed by dissolving 20 g. of corn starchin 40 cc. of electrolyte prepared as in Solution #2.

The starch and starch concentration employed have a significant effecton the resulting bibulous coating. For example corn starch tends toproduce a harder coating than wheat starch while yucca starch produces aharder coating than corn starch. A hard coating is desirable when theanode is to be pushed into place through a mass of battery mix duringcell assembly since the hard coating will not easily be dislodged orscratched off in spots by being pushed through the battery mix. Aconcentrated starch solution tends to produce a thicker bibulouscoating. In general a thick coating is undesirable since it will be morediflicult to push through the mass of battery mix and since it will takeup space which could otherwise be occupied by battery mix.

The prepared solution should be maintained at a temperature at which thestarch will not gel from the heat transferred thereto from the heatedanodes immersed therein. The required temperature will vary with suchfactors as the anode temperature, the time duration of anode immersion,the rate of anode immersion and the quantity of solution provided. Thetemperature of the solution should not be allowed to reach a value atwhich the starch will gel. This can best be prevented by cooling thesolution to. a suitable tempe rature,.e.g., and maintaining the solutionat this-temperature.-

Before immersion in the cooled solution, the anode should be heated to atemperature at least equal to that at which a starch coating'willformthereon and not greater than a temperature at which the solutionwill boil when in contact with the treated anode. Boiling causes bubblesto form in the coating. In general, the anode should be heated to atemperature between about 100 and 200 C. The anode, while still at atemperature in this range, should be immersed in the solu- Immersion ofthe heated anode in the starch solution 1 causes the starch in contactwith the anode to gel on the surface of the anode. This gel is a hardcoating which adheres closely to the metal and provides the electricallyinsulating porous surface desired for a primary battery anode. starchcoating.

The time of immersion of the anode in the solution should be relativelyshort, usually between about second and 2 seconds and preferably lessthan about 1 The gel is in the nature of a cooked or baked second. Toolong an immersion tends to produce toothick a coating which isundesirable, as observed'previously. Too long an immersion time alsotends to heat the electrolyte excessively. The dip time should, of

course, be sufliciently long to permit a coating of SHfficient thicknessto be formed, 'butunder proper co-ndi-' tions may be less than A second.

The bibulous coating, after formation, should bedried and then kept dryuntil assembly of the coated anode in. the pr-imary battery because thestarch will tend to absorb moisture from the atmosphere and become soft.

Initial drying may be effected by placing the coated anode in a hot dryatmosphere. An oven may be used to speed drying.

While various coating thicknesses can be used, a.,,

thickness of the order of ,4, to will usually be satisfactory. However,a thickness of has been successfully used. If desired, somewhatgreaterthicknesses may be used, e.g., V or more, but these greaterthicknesses are generally undesirable because of the space taken upthereby in the battery.

When battery assembly requires the coated anode to be pushed through amass of battery mix, it will, in most cases, be desirable to provideprotection for the coated leading edges of the anode, i.e'., the edgeswhich form 3 the opening in the mass of mix through which the remainderof the anode is pushed. The leading edges may be protected by a smallpiece of paper folded over the edges. Another alternative is to coat theleading edges with an insulating varnish or a suitable resin.

. It is desirable that the edge protection be easily applied andtherefore'a protective coating which can be applied by a simple dip,such as the insulating varnish, is preferred. No substantial portion ofthe sides of the anode structure need be protected in this way.

The solutions described previously have been formedwith natural starchessuch as corn starch, wheat flour, wheat starch and yucca starch. Thereare, however, synthetic gel forming materials which will form a similargel coating upon contact with the heated anode sur- 4 will not bedissolved upon insertion of the coated anode in the :battery mix. Suchsynthetic gel forming materials have heretofore been'used in primarybattery construction either alone or in combination with starch. It hasbeen found that the gel coating forming method of the invention isapplicable to such synthetic gel forming materials, although starch ispreferred for a bibulous coating. Contact of the heated anode structurewith a solution containing such a synthetic gelforming material producesa suitable bibulous coating for electrical insulating and ion transferpurposes, which coating can be used in place of the starch coatingpreviously described. Under some circumstances it will be desirable toinclude in the solution both starch and a synthetic gel formingmaterial, for example, in equal quantities by weight. Some synthetic gelforming materials may not be suitable for providing a bibulous coatingsince they may be dissolved by the electrolyte solution contained in the:battery mix. An example of a synthetic gel forming material which canbe used either alone or in combination with starch, in the practice ofthe invention is methyl cellulose. The use of such suitable syntheticgel forming materials is intended to be included within the scope of theappended claims and it should be understoodthat such materials areembraced within the word starch used therein. While the invention hasbeen described in connection with specific examples thereof and inspecific uses, variousmodifications thereof will occur to those skilledin the art without departing from the spirit and scope of the inventionas set forth in the appendedclaims.

' What is claimed is:

1. The method of forming a bibulous coating on a 'metallic anode elementwhich is subsequently to be assembled into a primary battery, comprisingthe steps of forming a starch containing solution, heating said anodeelement to a temperature lying within the range bounded by -'the lowtemperature at which the heated anode in contact with the starch in saidsolution will cause said starch to gel and the temperature at which saidsolution Will-boil when in contact with said heated anode, im-

mersing said heated anode element in said solution for a time intervaljust sufficient to form a gel coating of desired thickness on saidanode, and maintaining said solution at a temperature at which the heattransferred thereto from said anode element is insuflicient to raise thetemperature of said solution, except where said solution is in contactwith said anode element, to a value at which the starch in said solutionwill gel.

2. Themethod of forming a bibulous coating on a metallic anode elementwhich is subsequently to be assembled into a primary battery, comprisingthe steps of forming a solution containing a quantity of starchsufficient to gel upon contact with a heated anode and a quantity ofelectrolyte of the type to be used in the battery mix of said primarybattery, heating said anode element to a temperature lying within therange bounded by the low temperature at which the heated anode incontact with the starch in said solution will cause said starch to geland the temperature at which said solution will boil when in contactwith said heated anode, immersing said heated anode element in saidsolution for a time interval just suflicient to form a gel coating ofdesired thickness on said anode, and maintaining said solution at atemperature at which the heat transferred thereto from said anodeelement is insufficient to raise the tem ,face and which coating'isrelatively insoluble in the I electrolyte used in the battery mix, i.e.,the gel coating peratu're of said solution, except where said solutionis in contact with said anode element, to a value at which the starch insaid solution will gel.

3. The method set forth in claim 1 in which said time interval is withinthe range of about A second to 2 seconds.

4. The method set forth in claim 1 in which said time I interval is lessthan about 1 second.

5. The method set forth in claim 1 in which said solution is maintainedat a reduced temperature.

6. The method set forth in claim 5 in which said reduced temperature isof the order of C.

7. The method of forming a bibulous coating on a metallic anode elementwhich is subsequently to be as sembled into a primary battery,comprising the steps of forming a starch containing solution, heatingsaid anode element to a temperature lying within the range of about 100C. to 200 C., immersing said heated anode element in said solution for atime interval lying within the range of about A second to 2 seconds, andmaintaining said solution at a reduced temperature at which the heattransferred thereto from said anode element is insufficient to raise thetemperature of said solution, except Where said solution is in contactwith said anode element, to a value at which the starch in said solutionwill gel.

8. The method of forming a bibulous coating on a metallic anode elementwhich is subsequently to be assembled into a primary battery, comprisingthe steps of forming an electrolyte solution of the type to be used inthe battery mix for said primary battery, dissolving a quantity ofstarch in said solution, said quantity of starch being sufficient to gelupon contact with a heated anode, heating said anode element to atemperature 1ying within the range of about 100 C. to 200 C., immersingsaid heated anode element in said starch-containing solution for a timeinterval sufiicient to form a gel coating of desired thickness on saidanode element, and maintaining said starch-containing solution at areduced temperature at which the heat transferred thereto from saidanode element is insufficient to raise the temperature of said solution,except where said solution is in contact with said anode element, to avalue at which the starch in said solution will gel.

9. The method set forth in claim 8 in which said time interval is of theorder of about A second to 2 seconds.

10. The method set forth in claim 8 in which said time interval is lessthan about 1 second.

11. The method set forth in claim 8 in which said reduced temperature isof the order of 10 C.

12. The method set forth in claim 8 in which said desired thickness isless than about inch.

13. The method of forming a bibulous coating on a metallic anode elementwhich is subsequently to be assembled into a primary battery, comprisingthe steps of forming a solution containing a quantity of starchsufiicient to gel upon contact with a heated anode element, heating saidanode element to a temperature lying within the range of about C. to 200C., immersing said heated anode element in said solution for a timeinterval less than about 1 second, maintaining said solution at areduced temperature at which the heat transferred thereto from saidanode element is insufiicient to raise the temperature of said solution,except where said solution is in contact with said anode element, to avalue at which the starch in said solution will gel, drying the gelcoating formed on said anode element, and maintaining said gel coatingdry until assembly thereof into said primary battery.

14. The method set forth in claim 13 in which the thicknes of said gelcoating is of the order of 5 inch or less.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF FORMING A BIBULOUS COATING ON A METALLIC ANODE ELEMENTWHICH IS SUBSEQUENTLY TO BE ASSEMBLED INTO A PRIMARY BATTERY, COMPRISINGTHE STEPS OF FORMING A STARCH CONTAINING SOLUTION, HEATING SAID ANODEELEMENT TO A TEMPERATURE LYING WITHIN THE RANGE BOUNDED BY THE LOWTEMPERATURE AT WHICH THE HEATED ANODE IN CONTACT WITH THE STARCH IN SAIDSOLUTION WILL CAUSE SAID STARCH TO GEL AND THE TEMPERATURE AT WHICH SAIDSOLUTION WILL BOIL WHEN IN CONTACT WITH SAID HEATED ANODE, IMMERSINGSAID HEATED ANODE ELEMENT IN SAID SOLUTION FOR A TIME INTERVAL JUSTSUFFICIENT TO FORM A GEL COATING OF DESIRED THICKNESS ON SAID ANODE, ANDMAINTAINING SAID SOLUTION AT A TEMPERATURE AT WHICH THE HEAT TRANSFERREDTHERETO FROM SAID ANODE ELEMENT IS INSUFFICIENT TO RAISE THE TEMPERATUREOF SAID SOLUTION, EXCEPT WHERE SAID SOLUTION IS IN CONTACT WITH SAIDANODE ELEMENT, TO A VALUE AT WHICH THE STARCH IN SAID SOLUTION WILL GEL.